Add codecov to Github Action CI

[danielhuppmann]

Description of PR

This PR adds pytest-cov and upload to Codecov to Github action (py3.8).

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@3896553). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #407   +/-   ##
=========================================
  Coverage          ?   86.29%           
=========================================
  Files             ?       35           
  Lines             ?     4051           
  Branches          ?        0           
=========================================
  Hits              ?     3496           
  Misses            ?      555           
  Partials          ?        0           

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